The present invention relates to an apparatus for controlling the liquid withdrawal from a dialysis circulatory system in connection with a hemodialysis treatment operating as an artificial kidney. This apparatus shall be suitable as a component of a complete dialysis apparatus and also as an attachment to devices which heretofore have been operated without controlling the liquid withdrawal from the circulatory system.
As is known, a hemodialysis treatment comprises mainly the step of guiding the blood of the patient to be treated along a surface of a semi-permeable membrane while an aqueous solution of a suitable composition flows in a counter current flow thereto on the other side of the membrane. Substances which normally ought to be discharged through the kidneys, enter by diffusion from the blood through the membrane into the dialysis solution and are removed along with the latter. Besides such removal of urine bound or urenic substances, it is also necessary to withdraw from the patient whose kidney function has stopped, a certain quantity of water, in order to normalize the patient's liquid balance.
In substantially all of the hemodialysis devices which have been used heretofore, said liquid withdrawal is adjusted by the presetting of a pressure difference between the blood side of the semi-permeable membrane and the dialysis solution side of the membrane, whereby the fact is utilized that an increasing liquid quantity passes through the membrane with an increasing pressure difference across the membrane. However, the ultrafiltration characteristic of the membranes, that is, the relationship between the pressure across the membrane and the liquid passage therethrough per unit of time is subject to substantial variations from membrane specimen to membrane specimen. Said relationship is further influenced by the time changes in the membrane permeability due to deposits of blood components so that for these reasons alone said method exhibits large inaccuracies. In addition, the influence of the fluctuations of the blood pressure must be taken into account since these fluctuations cause corresponding fluctuations of the pressure across the membrane.
A frequently used method for controlling the liquid withdrawal comprises continuously monitoring the weight of the patient during the treatment by means of an in-bed scale. This operation, however, involves a substantial effort and expense because the price of an in-bed scale is approximately of the same order as the price of a hemodialysis apparatus. Thus, different suggestions have been made and some of them have been carried into practice, which aim at integrating the measuring or the control of the liquid withdrawal into the function of the hemodialysis apparatus.
One of these prior art methods makes it possible to indicate the actual value of the rate of liquid withdrawal by actuating a selector switching device. This is accomplished in that the influx of the dialysis solution is shut off while maintaining the pressure across the membrane and inserting into the discharge pipe a flow meter from which the instantaneous liquid withdrawal rate may be read. However, a continuous measuring is not possible according to this principle because it requires an interruption of the normal operation.
Another known method provides filling the entire dialysis solution supply into a closed, rigid storage container and to return the used-up dialysis solution into the same storage container. The desired liquid quantity per unit of time is withdrawn from this system having a constant volume, by means of a pump having an adjustable feed rate. Due to the fact that the total system is unyielding, the withdrawn liquid quantity must correspond to the liquid quantity which entered from the blood through the membrane into the dialysis solution circuit. An essential disadvantage of this method resides in the fact that the available volume of dialysis solution is limited to the volume of the storage container.
Another disadvantage of the just mentioned prior art is the fact that the efficiency decreases constantly during the treatment because fresh and used-up dialysis solution intermix.
The method described in the just preceding paragraph has been further developed in that a smaller container is used while working in accordance with the same principle and periodically renewing the content of the smaller container at relatively short time intervals. This is accomplished by quickly emptying the container after a predetermined length of time and newly filling it, whereby a buffer container is connected in series in order to keep available a sufficient liquid volume for the rapid filling. However, the loss of time resulting from the emptying and filling of the container can, nevertheless, not be reduced at random and constitutes a disadvantage in any event.
Other methods aim at providing a "dynamically rigid" system in order to withdraw liquid from the system in a defined manner as described above with regard to the rigid container. This may be accomplished in that with suitable technical means a conformity between the through flow rates in the supply and discharge of the dialysis solution is enforced which conformity is as exact as possible. In principle this may take place, for example, by means of two synchronized pumps in the supply and discharge conduits. However, the requirements regarding the accuracy are very high and the resulting technical expenditure is substantial.